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PostPosted: Sun Nov 22, 2015 8:27 pm 
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I can't find any information about it on the internet. I want to play Japanese and European games on my USA Sega CD model 2. I know it can be achieved by flashing region free BIOS file into cartridge and inserting it into Genesis cartridge slot. As far as I know burning an EPROM into regular cartridge does NOT work, I tried it myself and I can confirm it doesn't work. I've heard that the cartridge requires something else to be added to get this to work, some additional chip or something

So, does anyone know how to make a Sega CD region free BIOS cartridge?? Any suggestions/advices will be strongly appreciated!! Many thanks in advance!!


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PostPosted: Mon Nov 23, 2015 1:46 am 
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everdrives can do this. i've never heard of a (homebrew) standalone cart for this purpose but that would be an interesting idea.


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PostPosted: Mon Nov 23, 2015 4:49 am 
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Location: Estonia, Rapla city (50 and 60Hz compatible :P)
You need a bit of active hardware to do it. Something that watches the bus and then plays with the !CARTIN signal to prevent bus fights with MegaCD side when the BIOS area is accessed. If you can disable the BIOS chip in the MCD you will not have to deal with any active logic and run ROM from cart directly (and leave the !CARTIN open).

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Last edited by TmEE on Mon Nov 23, 2015 7:48 am, edited 1 time in total.

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PostPosted: Mon Nov 23, 2015 5:17 am 
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There is already a thing that you can buy:
http://products.genny4ever.net/megacart_v1x.html

Well first of all, I'm kind of cheap and don't want to spend 70$ on something that I am able to do myself anyway. As you mentioned before, even Everdrives can achieve this so this is possible to do through cartridge slot but can't find any information how this works....


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PostPosted: Mon Nov 23, 2015 7:49 am 
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Location: Estonia, Rapla city (50 and 60Hz compatible :P)
I mentioned how it is done and an alternative way to go about it. You toggle !CARTIN in signal on BIOS accesses to prevent bus fights or you disable BIOS ROM in the Mega CD and run BIOS from cartridge that has !CARTIN signal open.

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PostPosted: Mon Nov 23, 2015 8:18 am 
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I don't want to modify any console's hardware, so disabling internal Sega CD BIOS is out of question. The only way I want to do it is through cartridge slot then

Seems like you know what you're talking about, so do you know what exactly happens while Sega CD wants to read internal BIOS, how !CARTIN signal works etc.?? I've only swapped EPROMs in Sega game cartridges so I don't know how the rest of hardware works and how I should manipulate it

Also, should I be concerned about Genesis memory map??
http://www.db-elec.com/home/technical-i ... memory-map

Now I noticed that regular carts are addressed in first 4MB part of address bus, 0x000000-0x3FFFFF and the next 4MB part is addressed by Sega CD. But, will it work if I add some kind of demultiplexer so the BIOS from cartridge will be read at 0x400000=0x7FFFFF?? Or am I doing it completely wrong??


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PostPosted: Mon Nov 23, 2015 6:17 pm 
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Location: Estonia, Rapla city (50 and 60Hz compatible :P)
!CARTIN signal swaps the 000000-3FFFFF(cartridge) and 400000-7FFFFF(expansion) ranges. When there is no cart inserted (!CARTIN = 1) the ranges are swapped : 000000-3FFFFF becomes expansion and 400000-7FFFFF becomes cartslot range. MegaCD BIOS is ALWAYS visible in the expansion range, you cannot turn it off.

The general idea is that all BIOS ROM access from the cartridge should have !CARTIN at 0 so that MegaCD BIOS is safely out the way, and all accesses that are not BIOS ROM related (RAM banks and such) will push !CARTIN to 1 to put the hardware in right place for the ROM code.

I don't remember how much of the expansion range is occupied by the MCD BIOS, I recall it was mirrored a few times but that's it.

The hardware should be pretty straightforward, when certain amount of top address bits are in a specific state you put CARTIN to one state, otherwise to another.

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